"Our extrapolation assumes that sea level continues to change in the future as it has over the last 25 years. Given the large changes we are seeing in the ice sheets today, that is not likely," he added.PTI Washington | Posted : Feb 14 2018 1:29AM | Updated: Feb 14 2018 12:02AM

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The global sea level is not rising at a steady rate, it is accelerating a little every year, according to a new assessment based on 25 years of satellite data.

Researchers at University of Colorado at Boulder in the US calculate that the rate is increasing by about 0.08 millimetres per year (mm/year) - which could mean an annual rate of sea level rise of 10 mm/year, or even more, by 2100.

"This acceleration, driven mainly by accelerated melting in Greenland and Antarctica, has the potential to double the total sea level rise by 2100 as compared to projections that assume a constant rate - to more than 60 cm instead of about 30," said Steve Nerem, a professor at the University of Colorado Boulder.

"And this is almost certainly a conservative estimate," Nerem said.

"Our extrapolation assumes that sea level continues to change in the future as it has over the last 25 years. Given the large changes we are seeing in the ice sheets today, that is not likely," he added.

If the oceans continue to change at this pace, sea level will rise 65 cm by 2100 - enough to cause significant problems for coastal cities, according to the assessment published in the journal PNAS.

Rising concentrations of greenhouse gases in Earth's atmosphere increase the temperature of air and water, which causes sea level to rise in two ways.

First, warmer water expands, and this "thermal expansion" of the oceans has contributed about half of the 7 cm of global mean sea level rise we have seen over the last 25 years, Nerem said.

Second, melting land ice flows into the ocean, also increasing sea level across the globe, he said.

These increases were measured using satellite altimeter measurements since 1992, including the US/European TOPEX/Poseidon, Jason-1, Jason-2, and Jason-3 satellite missions.

Nerem and his team used climate models to account for the volcanic effects and other datasets to determine the ENSO effects, ultimately uncovering the underlying sea-level rate and acceleration over the last quarter century.

They also used data from the GRACE satellite gravity mission to determine that the acceleration is largely being driven by melting ice in Greenland and Antarctica.

"This study highlights the important role that can be played by satellite records in validating climate model projections," said John Fasullo, a climate scientist at the National Center for Atmospheric Research.

"It also demonstrates the importance of climate models in interpreting satellite records, such as in our work where they allow us to estimate the background effects of the 1991 eruption of Mount Pinatubo on global sea level," said Fasullo.